Factors influencing precision and accuracy of a carbon-in ash monitor

This research is to enhance the performance of a carbon-in-ash monitor based on the photoacoustic effect. The original instrument was invented at ISU and licensed to Ametek Corporation for commercial manufacture. However, the company encountered problems with precision and accuracy of measurements performed with prototype instruments. The purpose of this study is to investigate factors that influence the precision and accuracy of photoacoustic measurement of carbon in fly ash.; Measurements are influenced by both fly ash sampling and preparation procedure and operational features of the instrument. Thermogravimetric analysis (TGA) tests have shown that as-received fly ashes are heterogeneous and need to be ground for the calibration of the carbon-in-ash-monitor. Homogeneity of fly ash is the most important factor to affect the precision and accuracy of the carbon-in-ash monitor. A sample preparation procedure was standardized to control sample density and to eliminate operator-introduced errors. Thermogravimetric analysis tests have also shown that LOI reports high for unburned carbon in fly ash because it accounts the weight loss from the release of compounds decomposed under high temperature. The highest relative error of LOI as a carbon indicator in the seventy fly ash samples collected from four countries is 3040%. Total organic carbon (TOC) measured with TGA was proven to be a correct carbon indicator and was used for the calibration of the carbon-in-ash monitor. It was found that photoacoustic signal saturation existed in the calibration process of carbon-in-ash monitor. Calcium carbonate was used to dilute samples to avoid the photoacoustic signal saturation. Universal calibration of the carbon-in-ash monitor was proven to be impossible but its custom calibration was successful. Redesigned head and LED holder of the carbon-in-ash monitor and increase of power output of excitation sources were evaluated to increase the responsitivity of photoacoustic signal for the improvement of accuracy of instrument calibration. Finally, statistics program was employed to study the effect of ambient temperature on precision and experiments with controlled ambient temperature confirmed the conclusion derived from the output of the statistic program.